Many high speed computers utilize some form of disk drive technology for information storage. Most commonly, hard disk drives (also known as rigid disk drives) are employed as the main memory resource. In a rigid disk drive, one or more disks rotate at a constant angular speed, with concentric data tracks being recorded on their surfaces. Transducers, also known as recording heads, are positioned over the desired data track by means of an actuator to either read or write information onto the disk surface. In this type of data storage system, information is recorded as a series of magnetic transitions stored on the surface of the disk medium.
The current trend in the computer industry is to provide portable computers which can be powered for extended periods of time using a battery. These computers are frequently known as laptop computers, notebook computers, hand-held computers, and like. One of the challenges in producing a computer which can be easily transported by its user is the management of system power. That is, the goal is to be able to conserve the average power consumed by the device and thereby prolong battery life. In the past, considerable attention has been focused on developing low power circuit designs in order to reduce the amount of standby power consumed by the computer's microprocessor and related circuitry. In these designs various processor functions are disabled or turned off when they are not required or in current use. Few designs, however, have addressed the need for conserving power used by the computer's disk drive unit.
Disk drives generally utilize complementary metal-oxide semiconductor (CMOS) technology for generating the external clock signal required by the disk drive unit, as well as the internal circuitry utilized for reading and writing information to the magnetic disk. Although CMOS circuits minimize power consumption in quiescent or study state conditions, power consumption can still be quite high during switching transitions. For example, the primary power consumption in CMOS designs lies in the power consumed by the output drivers during logic level changes. Especially problematic is the fact that power consumption of CMOS oscillating clock circuits increases with higher frequencies.
In the past, practitioners have used low power CMOS crystal oscillator designs for generating the clock signal required by the disk drive. However, because the clock signal required can typically be on the order of several hundred megahertz, prior art designs have generally employed crystals using higher (i.e., overtone) harmonics. By way of example, these crystal oscillators usually operate at overtone frequencies greater than 16 MHz. As noted above, the power consumption of a CMOS crystal oscillator increases with increasing frequency. For instance, third overtone crystal oscillators are typically needed to generate the high frequency clock signals required by most modern disk drives.
Another drawback of disk drives that utilize overtone mode crystals, is that they suffer from low efficiency. In addition, overtone mode crystal oscillators require more elaborate matching circuitry which further reduces their power efficiency. These inefficiencies result in power requirements that exponentially increase with frequency. Obviously, providing such high frequency clock signals to the disk drive unit on a continual basis means that a great deal of power is consumed due to the nature of CMOS circuitry, and also because of the inefficiency of conventional oscillator circuitry. Therefore, what is needed is an apparatus for conserving power in a disk drive unit incorporated into a portable computer.
As will be seen, the present invention provides a clock generating circuit which utilizes a crystal oscillator circuit operating at a fundamental frequency to minimize power consumption. In addition, the present invention incorporates circuitry for reducing the frequency of the clock signal provided to the disk drive unit when in a stand-by or low power mode of operation.